1. Field of the Invention
The present invention relates to a method for disinfecting drainage used for washing filtration membranes for purifying raw water such as groundwater or river water.
2. Description of the Related Art
Heretofore, in order to produce tap water or industrial water from raw water such as river water, lake water, and groundwater, filtration membranes have been widely used on their merits such as reliability and automatic operation. However, when water treatment equipment including a filtration membrane is operated for a long time, suspended solids in raw water adhere to the surface of the filtration membrane, which results in the decrease of the water permeation rate and water treatment capacity. More specifically, accumulation of solid contents and microorganisms (for example, Cryptosporidium, Giardia, fish eggs, algae, and Escherichia coli) increases the filtration pressure, which results in the decrease of the treatment capacity. Therefore, water purification with a filtration membrane requires backwashing or other operation every 30 minutes to 1 hour thereby washing the surface of the filtration membrane to remove the adhering suspended solids. As a result of this, drainage used for washing the filtration membrane contains suspended solids derived from the raw water at relatively high concentrations.
As described above, drainage used for washing filtration membranes contains various suspended solids derived from the raw water, and the suspended solids may contain concentrated infectious microorganisms, such as Cryptosporidium or Giardia which will not be deaden by a diluted chlorine sterilizer.
Heretofore, drainage used for washing filtration membranes is released untreated. Such discharge of wastewater containing infectious microorganisms is undesirable because it can cause recontamination of river water and others. In addition, water filtered through a filtration membrane is often used for backwashing, so that the discharge of drainage used for backwashing is undesirable from the viewpoint of effective utilization of raw water.
In order to solve the above-described problems, for example, JP-A KOKAI No. 11-90432 (Patent Document 1) suggests a method for water disinfection, including separating and concentrating suspended solids in washing drainage, and then disinfecting the concentrated water containing the separated suspended solids through, for example, ultraviolet irradiation method or heating method. JP-A KOKAI No. 2004-66061 (Patent Document 2) suggests another method for water disinfection, including reserving drainage in disinfection treatment equipment disposed out of a filtration membrane module, and disinfecting the drainage by disinfection treatment means such as ultraviolet light, followed by discharge.
However, the above-described disinfection methods for washing drainage described in Patent Documents 1 and 2 present the following problems.
Under the method described in Patent Document 1, the concentration of the suspended solids containing infectious microorganisms in the concentrated water is so high that transmission of ultraviolet light is inhibited, which results in the deterioration of the disinfection efficiency to inhibit reliable disinfection treatment.
Under the method described in Patent Document 2, suspended solids containing infectious microorganisms after separation and concentration with a filtration membrane are contained in the washing drainage at high concentrations, so that transmission of ultraviolet light through the water is inhibited, and the treated water is discharged in an insufficiently disinfected state, which may results in incomplete prevention of recontamination of rivers and the like. In addition, under the method, the drainage used for washing a filtration membrane is wholly discharged, which is undesirable from the viewpoint of effective utilization of water resources.